The present invention will be mainly described in the context of an implementation of the invention for a cardiac pacemaker as an example. It should be understood, however, that the invention is applicable far more generally to a wide variety of active implantable medical devices and active medical devices which are not implantable devices, for example, devices carried externally by the patient. In these active medical devices, the cardiac activity is collected (sensed) at the input of electrodes and the obtained signal is applied to an amplification and filtering module.
The amplifier portion of the module is generally foreseen to receive signals having an amplitude on the order of a millivolt in a frequency band spreading over a range typically from 1 Hz to 80 Hz. However, the more recent devices detect signals outside these limits for processing. Indeed, for example, typical VDD pacemakers present lower sensitivities, on the order of 0.1 mV, because they use in the atrium a floating electrode for the collection of atrial depolarizations (P waves) and, to collect these signals, it is necessary to increase the gain of the amplifiers.
Furthermore, current pacemakers typically possess the so-called "Holter" functions, that is to say the memorization (storage) and analysis of the cardiac activity over a very long period, typically several hours. The analysis of the endocardiac signal that is operated to this aim necessitates a band-pass whose minimal (lower) frequency is far lower than the normal 1.0 Hz limit, typically 0.1 Hz, to be able, for example, to analyze the ST segment of the collected cardiac signal.
These improvements, however, entail the appearance of new problems, particularly due to the fact of the band-pass being larger in the low frequency domain.
First of all, independently of its sensitivity, the input amplifier has to be able to support the high amplitude of the stimulation pulse to be applied by the cardiac pacemaker device, which pulse amplitude can reach 10 V, and then to recover as fast as possible its capacity to detect (sensitivity) signals on the order a millivolt.
To be able to support the high voltage of the stimulation pulse, it is always foreseen to have a period of "blanking" (disconnection) of the input circuits at the moment when the pulse is delivered. Nevertheless, at the end of the blanking period, the moment when the amplifier of input is again commuted (connected) for collecting signals, a large saturation of the input stage can occur because the potential of the heart/electrode interface has not returned to its rest value.
This problem is further aggravated by the fact that the time of recovery of the amplifier becomes longer as the cut-off frequency of the high-pass filter of the input stage becomes lower. Thus, for a high-pass filter cut-off at 0.1 Hz, the recovery time of the amplifier is on the order of 10 s, which is totally incompatible with the need that one has to react rapidly to the changes in the detected cardiac signal.
It is indeed possible to apply the so-called technique of "pre-or post-charge", which concerns delivering electrical charges before or after the stimulation pulse to compensate for the lengthening of the recovery time. Nevertheless, this method is a large consumer of energy and would not be applied in a permanent manner without decreasing notably the duration of the life of the implanted device.
Another problem resides in the size of capacitor and resistor components, which increase as the cut-off frequency of the high-pass filter at the input stage is reduced. One sees that the more that the "listening" window widens in the low frequency domain (i.e., the frequency range used in the spectral analysis), it becomes necessary to have filter components of greater size, which is incompatible with the design imperatives of circuit miniaturization for implantable devices.